Highly conducting polypyrrole (PPY) films, doped with various anions [pTS À , ClO 4 À , and NO 3 À and mixed electrolyte system (pTS À þ ClO 4 À )], have been electrochemically synthesized in aqueous solution at $275 K in an inert atmosphere. PPY exhibits metallic order dc conductivity at room temperature and shows variation of conductivity with respect to time of polymerization. Effect of dopant anion on growth mechanism of PPY is evident from its surface morphology. X-ray photoelectron spectroscopy (XPS), used to examine the surface composition and doping level of various PPY films, confirms the anionic doping into the polymer backbone. Both XPS and ultraviolet-visible spectroscopy give evidence of formation of polarons and bipolarons. The temperature (4.2-320 K)-dependent dc conductivity data of these PPY films have been explained by Mott's 3D variable-range hopping conduction model. Mott's parameters have been estimated, and structural disorder with doping is correlated for all the samples. Mott's criterion for distant hopping sites prevails in case of moderately doped samples (PPY3, PPY4, and PPY5), whereas the hopping to nearest neighbor sites is found more suitable in case of highly doped samples (PPY1 and PPY2). The origin of these changes is due to the modification in the molecular structure of PPY, which is governed by different growth mechanisms for organic (pTS À ) and inorganic (ClO 4 À and NO 3 À ) counter anions. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 347-360, 2012
The present study reports a strong thickness-dependence and anomalously large enhancement in the values of the Seebeck coefficient and electrical conductivity in Bi2Te3 films at ultralow thickness. An opposite sign of the Hall coefficient (negative) and Seebeck coefficient (positive) is observed in an ultrathin Bi2Te3 film (65 nm) as compared to the normally observed identical sign in the case of Bi2Te3 thin films (520 nm). A simultaneous enhancement in the values of electrical conductivity and the Seebeck coefficient results in a giant enhancement in the value of power factor from 1.86 mW/m K2 to 18.0 mW/m K2 at 416 K, with a reduction in thickness. X-ray photoelectron spectroscopy investigation reveals the absence of any significant change in stoichiometry and chemical bonding upon reduction of thickness. Magnetoresistance vs magnetic field data show a sharp dip at the lower magnetic field values, indicating a weak antilocalization effect in the case of the ultrathin film sample suggesting the role of strong spin–orbit coupling toward the carrier filtering effect resulting in enhancement of thermoelectric properties. Observation of the large Seebeck coefficient and the power factor at lower thickness values and its relationship with spin–orbit coupling is an important result, both for practical applications and for better understanding of the thermoelectric properties.
Electrochemically synthesized polypyrrole (PPy) films degrade under anodic polarization in aqueous solution. PPy is irreversibly oxidized leading to subsequently to an insulating material. This article highlights the degradation behavior of electrochemically synthesized PPy films using inorganic (perchlorate, ClO 4 À ), organic (p-toluenesulfonate, pTS À ) and mixed (inorganic-organic,anions systems in aqueous solution. The PPy films were subject to overoxidation by means of anodic polarization for a certain period of time and consecutively cyclic voltammetry and electrochemical impedance spectroscopy were employed to investigate the influence of anions used during polymerization on the stability and redox activity of the polymer. The modification in the surface morphology of PPy films due to the application of anodic polarization distinctively shows the effect of degradation at polymer/electrolyte interface. The presence of organic anion in the system (PPy/pTS and PPy/mixed) has shown improved electrochemical activity and is stable to electrochemical degradation for a longer period of time than PPy/ClO 4 À .
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